How does 2 people drown in a swimming pool ? Usually if somebody is drowning, the other can help. What are the odds of it being just an accident over a real murder then ?
You're assuming that they died at the same time. Just to think outside of the box for a moment (this was a murder mystery party, after all) what if the killer drowned the first victim and left them in the pool, waiting for the second victim to come and attempt to rescue victim 1? Victim 2 jumps in, and the killer uses the opportunity to surprise them and drown them next.
Also, there is no mention in the article that they died from drowning, or that they were conscious when in the pool. Someone could have knocked them out and put them in the pool for whatever reason, leaving the unconscious victims to drown.
I guess if you happened upon a car accident you'd just nonchalantly step off to the side and wait for the ambulance and such like to show up. Might be dangerous to help the people out of the car in case it explodes or something.
It wouldn't be nonchalantly, but I would probably step away and assess the situation. As others have said, as I don't have up-to-date medical training I would probably do more harm than good.